Hemoglobin (Hb), the oxygen-carrying protein in erythrocytes transports oxygen from respiratory organs such as respiratory tracts and lungs and releases oxygen to organs and peripheral tissues of a human body such that the organs and the peripheral tissues can be supplied with sufficient oxygen in order to maintain their normal physiological functions.
Hemoglobin of human adults is a tetramer α2β2 consisting of four subunits, α1, α2, β1 and β2, wherein each subunit relies on intermolecular interactions such as intra-subunit hydrogen bonds to sustain its secondary and tertiary structures. Additionally, the inter-subunit hydrogen bonds formed among the aforementioned four subunits allow the quaternary structure of hemoglobin to be formed.
Hemoglobin can reside in two different quaternary configurations, including the relaxed form (R form) having high oxygen affinity and the tense form (T form) having low oxygen affinity. When hemoglobin is travelled to lungs through the blood circulation, hemoglobin becomes bound with oxygen and resides in the R quaternary configuration of high oxygen affinity. The oxygenated hemoglobin is then transported to organs and peripheral tissues through blood circulation and releases oxygen to organs and peripheral tissues and transforms into the T quaternary configuration of low oxygen affinity. The allostery of hemoglobin is also influenced by several allosteric factors, such as the pH value, the carbon dioxide concentration and the 2,3-BPG concentration in erythrocytes.
2,3-bisphosphorglycerate (2,3-BPG, or 2,3-diphosphoglycerate (2,3-DPG), hereinafter “2,3-BPG”) is the endogenous allosteric effector of hemoglobin and the most important chemical species in an erythrocyte of a human body besides the oxygen-carrying entity, hemoglobin. 2,3-BPG delicately regulates the configuration of hemoglobin by interacting with the β1 and β2 subunits of hemoglobin to stabilize hemoglobin in the low oxygen affinity T form to reduce the oxygen affinity of hemoglobin, thereby facilitating hemoglobin to effectively release oxygen to body organs and tissue cells.
The conventional hyperbaric oxygen therapy is to place a patient in a hyperbaric chamber, wherein the chamber is pressurized and maintained at 2.0-3.0 atmospheres, while the patient inhales 100% oxygen through an oxygen mask to increase blood oxygen concentration, thereby improving the hypoxia, promoting wound healing or treating acute hypoxia conditions.
However, the hyperbaric oxygen therapy may cause considerable adverse side effects, including barotrauma, decompression sickness and oxygen poisoning, which can at times become fatal if improperly operated, because the patient inhales a high concentration of oxygen under the high pressure, which may lead to rapid pressure changes among tissues.